Hip joint prosthesis

ABSTRACT

A femoral implant for fitting to a resected femur comprises a section having a frustoconical external form, a stem part extending away from the section to a distal end of the implant, and a femoral head extending directly away from the section to the proximal end of the implant. The implant could have the femoral head integral with the section, or as a separate component secured thereto, with the stem being a separate one piece component incorporating the section.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S.application Ser. No. 12/889,541, which was filed on Sep. 24, 2010,entitled HIP JOINT PROSTHESIS and claims priority to U.S. applicationSer. No. 10/396616, now U.S. Pat. No. 7,879,106, which was filed on Mar.25, 2003, entitled Hip Joint Prosthesis and claims priority to GreatBritain Application No. 0207170.2, which was filed on Mar. 26, 2002,entitled HIP JOINT PROSTHESIS, the disclosures of which are incorporatedby reference herein in their entirety.

This invention relates generally to a hip joint prosthesis, and inparticular to a method of fitting a femoral implant to a resected femur,and to the femoral implant per se.

Hip replacements involve the use of an implant stem which is fitted intothe medullary canal of the femur. Generally such stems achieve optimumfixation when a tapered prosthesis 10 is fitted in the medullary canalof a tapering bone 11, as schematically shown in FIG. 1. Thisrequirement is generally achieved with a conventional type total hipreplacement stem 12 fitted into the proximal femoral portion 13 as shownin FIG. 2.

This form of treatment, however, suffers from the followingdisadvantages:

1) It requires extensive bone resection of the femoral head and part ofthe femoral neck.

2) It leads to loading of the shaft of the femur in a non-physiologicalfashion, leading to distal load transfer and proximal femoral shaftstress shielding.

3) There can be loosening of the total hip replacement stem, as there isa cantilever effect with the point of loading on the prosthetic femoralhead at a distance from the point of fixation in the proximal femoralshaft.

With young active patients it is now regular practice to insert a moreconservative type of hip replacement known as a resurfacing, and thistype of prosthesis overcomes in major part the three disadvantageslisted above. However some patients are not suitable for hipresurfacing, mainly on account of poor bone in the zenith of the femoralhead due to cystic destruction, as shown at 14 in FIG. 3. With youngpatients with such cystic destruction, there is a need for a moreconservative type of hip replacement than the standard variety shown inFIG. 2.

An example of a more conservative hip replacement is disclosed in EP0579868 A2. This type of implant attempts to conserve bone, load thefemur more proximally and, by retaining most of the patient's femoralneck, resist torsional loading more satisfactorily than the standardtotal hip replacement. However this type of prosthesis does not fulfillthe requirements referred to in relation to FIG. 1. In particular it canbe seen from EP 0579868 A2 that the bone in the femoral neck expandsinto a widened bone in the proximal femoral shaft (inter-trochantericregion), thus not fulfilling the FIG. 1 requirements.

An object of the invention is to provide an improved method of fitting afemoral implant to a resected femur and also an improved femoral implantitself and an improved femoral implant stem.

According to a first aspect of the invention there is provided a methodof fitting a femoral implant to a resected femur comprising resectingthe femoral bone through the femoral head, machining said resectedproximal end of the femur to form a frustoconical or generallyfrustoconical cavity in the bone, and inserting into said cavity afemoral implant, which in use extends into the medullary canal of thefemur.

Preferably the femoral bone is resected through the base of the femoralhead. Conveniently the cavity machined into the bone of the femur issymmetrical about a centre line thereof. Advantageously at or adjacentits proximal end the femoral implant is formed with a part having anexternal surface complementary to the surface of said cavity in thefemoral bone. Conveniently a curved stem part of the implant extendsaway from said part of the implant, and is received in said medullarycanal. Said stem part could however be straight.

According to a further aspect of the invention, a femoral implantcomprises adjacent a proximal end thereof a section having an externalsurface of frustoconical or generally frustoconical form, a stem partextending away from said section to a distal end of the implant and afemoral head extending directly away from said section to the proximalend of the implant.

Preferably the femoral head is integral with said section.Advantageously the femoral head is separate from said section.Conveniently the separate femoral head is secured to said section byrespective interfitting parts thereof. Desirably a frustoconical spigotextending from said section is received in a complementarily shapedcavity in the femoral head. More preferably the spigot is a tightfrictional fit in said cavity, or is cemented therein. The central partof the femoral head is preferably hollow. Desirably the implant ismanufactured of cobalt chrome. The stem part is preferably curved, butcould be straight.

According to a still further aspect of the invention a stem of a femoralimplant has a stem part extending to a distal end Of the stem and asection from which the stem part extends, said section having anexternal surface which is of frustoconical or generally frustoconicalform and which terminates at a free end of the section.

Conveniently a spigot extends from said free end of the section forconnection, in use, of a femoral head to said stem. Preferably saidspigot has a frustoconical external surface. Desirably the stem part iscemented in place or modified for suitable biological fixation, in use.The stem part is preferably curved, but could alternatively be straight.

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic view of an optimum fixation arrangement of afemoral prosthesis,

FIG. 2 is a similar view to FIG. 1, showing the fixing of a conventionaltype of total hip replacement stem,

FIG. 3 shows the proximal end of a femur, with three different positionsof the level of resecting shown at A-A, B-B and C-C respectively,

FIG. 4 is a view like FIG. 2 showing a femoral prosthesis of theinvention fitted by a method if the invention to a resected femur, theprosthesis including a stem also according to the invention, and

FIG. 5 is a view like FIG. 4, showing a one-piece femoral prosthesis.

To explain the present invention, reference is firstly made to FIG. 3which shows the proximal end of a femur 15. The femur has a head 16which is globular and forms rather more than a hemisphere. Connectingthe head with the main shaft 17 of the femur is a neck 18, thetransition between the head 16 and neck 18 being generally smoothlyconcavely curved as shown at 19. The superior border of the neckterminates at the great trochanter 20, whilst the inferior border of theneck terminates at the lesser trochanter 21.

In FIG. 3, the line A-A represents the resection level for aconventional total hip replacement, this section being taken towards theend of the neck furthest from the head 11. Line B-B represents theresection level for the prosthesis shown in EP 0579868 A2, this lyingthrough the neck at or adjacent transition area 19 between the head andthe neck of the femur.

With the present invention, as will be described, the resectioning isnot carried out at a level through the neck 18, but instead is carriedout through the femoral head. Accordingly the line C-C represents theresection level for one embodiment of the present invention. Byresecting along the line C-C through the base of the femoral head, bonein the transition area between the femoral head and the femoral neck isretained, thus providing the opportunity for exploiting the requirementsreferred to in relation to FIG. 1.

Accordingly the first stage in the hip replacement method is to resectthe proximal end of the femur at a level through the head 16, forexample at the line C-C. By resecting the bone at this position throughthe base of the femoral head, it is possible then to machine theinterior of the bone into the form of a truncated cone, i.e. in the formof an engine valve seating, with this frustoconical or generallyfrustoconical form of cavity produced thus for filling the requirementsset out in relation to FIG. 1. As shown in FIG. 4, the cavity 22 ispreferably symmetrical about a centre axial line E-E, and in thissymmetrical form the shape of the cavity 22 facilitates milling atsurgery. As will be appreciated, this machining which produces thecavity 22 is at the upper end of the medullary canal 23 which isgenerally curved as it extends downwardly from the neck into the mainshaft 17 of the femur 15.

FIG. 4 shows an embodiment of the invention in which the femoralprosthesis is formed in two parts, namely an implant part 24 which isreceived in and fitted into the femur and a femoral head component 25which is securely fitted to the implant part 24 as will be described.The implant part 24 has a curved stem part 26 which is received in andfollows the shape of the medullary canal 23, the part 24 thus definingthe distal end of the implant part 24. The stem part extends from asection 27 which has an external surface 28 shaped to match the internalsurface of the cavity 22, i.e. the external surface of the section 28 iscomplementary to the frustoconical or generally frustoconical internalsurface of the cavity 22 so as tightly to engage therein as shown inFIG. 4. It will be noted that the frustoconical form terminates at afree end of the section 27, in other words there is no transverselyprojecting collar or the like at the free end of the implant part 24 asis the case with the femoral rod shown in EP 0579868 A2. Integrallyextending from the free end of the section 27 is a central outwardlytapered circular spigot 29, this spigot enabling the femoral prosthesisto be of said two-part modular type with said separate femoral headcomponent 25 fitting onto the implant part 24 by virtue of the spigot 29engaging in a complementarily shaped central recess 30 of the femoralhead component 25. The connection between the spigot 29 and recess 30can be frictional, or alternatively some form of adhesive cement couldbe used. As shown in FIG. 4, the free periphery of the femoral headcomponent, when attached to the implant part 24, extends slightly overthe resected area of the femoral bone. In an alternative embodiment, thereverse arrangement could be employed, i.e. the fnistoconical spigotextends from the end of component 25 and enters a matching recess in theend of the section 27.

The prosthesis would typically be manufactured from cobalt chrome orother suitable metallic material, and the implant part 24 wouldgenerally either be cemented in place or modified for suitablebiological fixation. The head component 25 could either be used as ahemi-arthroplasty with the head component articulating against thepatient's normal acetabular cartilage or alternatively the headcomponent could be articulated against a prosthetic acetabular componenttypically manufactured from cobalt chrome, ceramic or polyethylenematerial. It will thus be appreciated that with the prosthesis describedand illustrated, the head component extends directly away from thesection 27 received in the cavity 22 in contrast to the arrangementdescribed and shown in EP 0579868 A2 where the head is connected via aconnecting portion to the implant part so that the head is spaced from,and thus does not directly extend from the free end of the implant partwhich, in this prior art arrangement is constituted by the transverselyprojecting collar. Typically for different patients, the maximumdiameter of the cavity 22, and thus effectively the maximum diameter ofthe section 27 received therein, is from 29 mm to 45 mm, and as shown inFIGS. 4 and 5, the cavity 22 at its maximum diameter extends across thewhole, or substantially the whole of the end surface of the resectedhead.

Accordingly, in accordance with a method of the invention the proximalend of the femur is resected at the level of the line C-C shown in FIGS.3 and 4, and the cavity 22 is then produced by machining, for examplemilling, this cavity being of frustoconical or generally frustoconicalshape. As explained above, this cavity 22 is formed at the upper end ofthe medullary canal 23 which extends in curved fashion from the line D-Dto T.

The implant part 24 is then fitted into the cavity 22 and medullarycanal 23 as shown in FIG. 4 and secured therein as described above. Thefemoral head component 25 could already be fitted on this implant part24 at the time of said insertion, or alternatively could be engaged ontothe spigot 29 once the implant part 24 has been fitted in place. Byvirtue of the extent of the section 27 fitting complementarily in itsmachining cavity 22, the advantages of EP 0579868 are achieved inconjunction with the requirements of FIG. 1 also being satisfied toprovide an improved prosthesis fixation. It will be understood that boththe femoral prosthesis, including a head and an implant, either integralor separate, as well as an implant part per se are considered novel andinventive in addition to the method of fitting, including saidresecting.

With regard to FIG. 5, this shows a further embodiment of a prosthesisfitted in place at the proximal end of a resected femur in a similarmanner to that shown in FIG. 4. The difference here is that theprosthesis 31 is formed in one piece with the central part of its headbeing hollow as shown at 32. As with the embodiment shown in FIG. 4, theprosthesis would be manufactured out of cobalt chrome or similarmaterial and the stem would be cemented in place, or modified forsuitable biological fixation. Moreover as with FIG. 4, the head would,as mentioned in relation thereto, either articulate against thepatient's normal acetabular cartilage or against a prosthetic acetabularcomponent manufactured from cobalt chrome, ceramic or polyethylenematerial. In a similar manner to that of FIG. 5, the separate femoralhead component 25 could have part thereof, for example a central partthereof, hollow.

In a further embodiment of the two-part prosthesis, the proximal end ofthe frustoconical aspect of section 27 could be extended to form aperipheral cone junction with the inner aspect of the femoral headcomponent. In a still further embodiment the stem part of the one or twopiece implant could be straight rather than curved.

The invention claimed is:
 1. A femoral implant comprising: a femoralhead component; a stem component having a first end and a second end,wherein said first end is configured to couple to said femoral headcomponent, wherein said second end is configured to be placed in afrustoconical or generally frustoconical cavity in a femoral bone of apatient, wherein said stem component comprises a diameter configured tobe substantially the same as the diameter of a resected end of a femoralhead of the femoral bone.
 2. The implant of claim 1 wherein the stemcomponent comprises a curved portion.
 3. The implant of claim 1 whereinthe stem component comprises a straight portion.
 4. The implant of claim1 wherein the stem component comprises a portion that is symmetricalabout a centre line thereof.
 5. The implant of claim 1, wherein the stemcomponent comprises a portion at or near the second end, said portionhaving an external surface complementary to the surface of said cavityin the femoral bone configured to tightly engage said cavity.
 6. Theimplant of claim 1, wherein the first end comprises a spigot configuredto couple to a recess in the femoral head component, said spigot andsaid recess comprise respective complementary shapes.
 7. The implant ofclaim 6 wherein the spigot is integral with the first end of the stemcomponent.
 8. The implant of claim 1, wherein the femoral head comprisesa periphery portion configured to extend over a portion of the resectedfemoral head when the implant is in use.
 9. A femoral implantcomprising: a femoral head component; a stem component configured to beplaced in a frustoconical or generally frustoconical cavity in a femoralbone of a patient, wherein said femoral head component is integral withsaid stem component; wherein said stem component comprises a maximumdiameter configured to extend across a maximum diameter of a cavity inthe femoral bone near a resected end , said maximum diameter issubstantially the same as the diameter of a resected end of a femoralhead of the femoral bone.
 10. The implant of claim 9 wherein the stemcomponent comprises a curved portion.
 11. The implant of claim 9 whereinthe stem component comprises a straight portion.
 12. The implant ofclaim 9 wherein the stem component comprises a portion that issymmetrical about a centre line thereof.
 13. The implant of claim 9,wherein the stem component comprises a portion having an externalsurface complementary to the surface of said cavity in the femoral boneconfigured to tightly engage said cavity.
 14. The implant of claim 9,wherein the stem component comprises a spigot configured to couple to arecess in the femoral head component, said spigot and said recesscomprise respective complementary shapes.
 15. The implant of claim 14wherein the spigot is integral with the stem component.
 16. The implantof claim 9, wherein the femoral head component comprises a peripheryportion configured to extend over a portion of the resected femoral headwhen the implant is in use.
 17. A femoral implant comprising: a femoralhead component; a stem component having a first end and a second end,wherein said first end is configured to couple to said femoral headcomponent, wherein said second end extends away from said first end;wherein said stem component comprises a diameter configured to besubstantially the same as the diameter of a resected end of a femoralhead of the femoral bone.
 18. The femoral implant of claim 17 whereinthe second end has a frustoconical or generally frustoconical shape. 19.The implant of claim 17, wherein the femoral head component comprises aperiphery portion configured to extend over a portion of the resectedfemoral head when the implant is in use.
 20. The implant of claim 17wherein the stem component comprises a curved portion.